The present invention is advantageously applied to the construction and operation of a radio frequency identification (RFID) tag such as that disclosed in commonly owned U.S. Pat. No. 5,530,702 of Palmer et al. ("the '702 patent"), and to an article-identifying transmitter in a checkout or inventory control system such as described in commonly owned U.S. Pat. No. 5,239,167 to Kipp ("the '167 patent"), for which U.S. reissue patent application Ser. No. 08/501,944 filed Jul. 12, 1995 is under allowance at the time this application is being prepared. Applicants incorporate the '702 patent and the '167 patent herein by reference.
The '167 patent discloses a system for detecting multiple randomly disposed articles without moving or contacting the articles. As described in the '167 patent, an actuator signals transmitters (RFID tags) attached to each article to begin operating. Each transmitter then transmits a signal identifying the article in turn and then deactuates itself.
Among the applications for which the system disclosed in the '167 patent is adapted are a checkout system and an inventory control system such as for a supermarket or warehouse. To be practical for a checkout system, RFID tags must be inexpensive to manufacture, because they are applied to merchandise articles with no requirement or expectation of being returned by the customer.
At the same time, the RFID tag must be capable of reliably transmitting a multiple digit identifying signal, and must operate in such manner that its transmissions will be received despite that many other RFID tags will also be operating and transmitting identifying signals.
In the checkout or inventory control systems disclosed in the '167 and '702 patents, RFID tags transmit and receive signals to and from an interrogator transceiver (hereinafter, "the interrogator") over relatively short distances on the scale of a few meters or less. Frequency modulation (FM) would be expected to provide outstanding communication performance in a system constructed according to the present invention. However, at the time this application is being prepared, the additional cost of equipping each RFID tag with an FM receiver as compared to the cost of an amplitude modulation (AM) receiver would not justify using an FM receiver in the present transponder invention. The inventors have found that amplitude modulation will provide adequate performance over the relatively short communication range for which the invention is primarily intended.
In view of the design choice which has been made, the range-limiting constraint becomes the sensitivity of the receiving circuitry of the RFID tag. In view of this range limitation, the inventors have found that a transponder implementation in which the RFID tag transmits by re-radiating a carrier frequency derived from a received carrier frequency further helps to satisfy both transmission performance and cost considerations.
In the communication system for which the present invention is adapted to operate, the RFID tag transponder transmits on a transmitting frequency which is derived from, but different from the frequency of a carrier signal which is broadcasted by the interrogator. As will be appreciated, the use of a different transmitting frequency by the RFID tag transponder facilitates the separation by the interrogator of signals transmitted by transponders from the signals and carrier frequency transmitted itself.
As to the method by which the transmitting frequency is generated, the provision of a local oscillator and mixer in the transmitting circuitry of the transponder would be expected to increase the noise rejection for signals transmitted by the transponder. However, the inventors have found that the added cost of such circuitry would provide no advantage because the range-limiting constraint remains the sensitivity of the receiver portion of the transponder.
By way of background to the present invention, U.S. Pat. No. 3,944,928 describes a harmonic communication system. As described in the '928 patent, a transponder receives a fundamental carrier frequency, generates a harmonic frequency therefrom, and modulates the harmonic frequency with a pulse-width modulating signal provided by a code storage 40 contained on the transponder.
U.S. Pat. No. 4,656,478 describes a transponder having an antenna 12 which is tuned to a broadcasted frequency f0 and a diode 19 which generates a second harmonic frequency 2.times.f0 therefrom for re-radiation on tuned antenna lobes 14a and 14b.
U.S. Pat. No. 4,979,232 describes a solid state switch which permits both a transmitter 45 and a receiver 55 to share a common antenna. As described in the '232 patent, reverse bias is applied to a diode 15 to present high impedance to block damaging currents generated by transmitter 45 from reaching receiver 55.
However, none of the '928 patent, the '478 patent, or the '232 patent describe a system by which a transponder suppresses the undesired emission of harmonic frequencies while not transmitting.
Finally, U.S. Pat. No. 4,059,801 describes a "de-Q-ing" circuit used to lower the Q factor of a transmitting antenna during quiescent intervals between transmissions. However, the '801 patent does not describe a system which affirmatively detunes an antenna to a different frequency to prevent a broadcasted frequency from being received thereon during non-transmitting intervals.
Accordingly, it is an object of the present invention to provide a transponder implementation of an RFID tag which is low in cost and operates reliably to communicate information over a transmission range of a few meters or less.
It is a further object of the invention to provide a transponder which transmits by re-radiating a harmonic frequency of a broadcasted carrier frequency and which contains circuitry for suppressing unwanted emissions of that harmonic frequency while not transmitting.